Drops, Bubbles and Wetting in Helium

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****NON-TECHNICAL ABSTRACT****Atoms near a solid surface experience a long range attractive force. Long range forces between molecules are important because they determine macroscopic properties of matter; such as what phase (solid, liquid, gas, etc) is stable at a given temperature and the friction between sliding surfaces. Long range forces also determine the shape of large biologically important molecules such as proteins and DNA. This individual investigator award supports project designed to measure long range forces in some simple model systems using several complementary methods. One method is to expose a solid to a gas and then weigh the amount of adsorbed gas on the surface using a microbalance that can detect fractions of a single atomic layer. Atoms adsorbed on a surface can be considered to be two dimensional matter. They undergo phase transitions such as vaporization and freezing, which are very similar to the familiar three dimensional counterparts. When the adsorbed atoms are very light and weakly interacting (e.g. helium), quantum mechanical effects become important and the adsorbed 2D liquid can also become a superfluid, which means that it can flow forever without any driving force. Theory suggests that for helium-4 on substrates of alkali metals (lithium, sodium, potassium, etc) there are unusual competitions between the liquid-vapor transition and the superfluid-normal fluid transition. This competition will be investigated using both microbalances and reflection of polarized light. Another way to measure surface forces is to use an atomic force microscope (AFM). The forces are measured by monitoring the deflection of a micromachined silicon cantilever with a laser when the cantilever is brought near the surface. Making these measurements with a cantilever immersed in liquid helium is technically challenging, and is one of the goals of this project. Students and post doctoral associates involved with this project will have the opportunity to develop novel instruments, and receive training in a broad spectrum of techniques including cryogenics, high speed imaging, laser optics, and materials preparation. This training will enable them to become productive members of the research community, whether in academia, industrial or government laboratories.****TECHNICAL ABSTRACT****This award supports an individual investigator project with several aims in the general area of fluids, predominately quantum fluids. Wetting and its relationship to superfluid phase transitions when films of helium and helium mixtures are adsorbed on weak and intermediate strength alkali metal and metal oxide substrates will be studied and compared with theory. For another study, low temperature atomic force microscope (AFM) techniques will be developed. The AFM will be optimized to enable measurement of the distance dependence of Casimir forces due to both fluctuations in the electromagnetic field and fluctuations in the order parameter near the onset of superfluidity. Finally a smaller project will investigate the pinch-off phenomena in drops and bubbles to elucidate the connections between the phenomena in conventional viscous fluids and non-Newtonian fluids. The computer code developed for the pinch-off studies will be adapted to study the stability and breakup of multi-electron drops in Helium. Furthermore the motion of superfluid droplets on non-wetting surfaces will be studied. Students and post doctoral associates involved with this project will have the opportunity to develop novel instruments, and receive training in a broad spectrum of techniques including cryogenics, high speed imaging, laser optics, and materials preparation. This training will enable them to become productive members of the research community, whether in academia, industrial or government laboratories.